There is a clause that some artists can only perform once within a certain region, within a certain amount of time. Everyone's contract details are different, but usually it is something like: "This artist can not perform more than once within a 30 day period at a venue within 50 miles of the previous venue." That is obviously in my own words, but is essentially the same thing. The reason for having a "Special Guest" listed on your flier, is that you can basically ignore that contract clause because you are technically not using that artists name to promote your event, and they are not the main attraction.
This is also done when the artist is good friends with the promoter and wants to give them a really good deal on DJ rates, but have a contract minimum. Rates can drop drastically this way. A $3,000 DJ could easily go down to a few hundred based on the right scenario. Most of the time, a "Special Guest" is a big name, but in some cases promoters will try to fool you into thinking they landed someone big, just to get you to come out. That is kind of a shady move, if not just ignorant. Sometimes an old friend, that moved to another state, may have come to town for a surprise visit, and the promoter wants to surprise everyone. That is cool and all, but it is very misleading because these days, everyone assumes "Special Guest" is a big headliner.
You may wonder why artists are forced to limit themselves, and I hate to say it, but much of the time it is because of fickle fans. People start to shit talk, or at least try to sound cool to their friends if they saw the performer recently already. You hear it all the time... "Oh, so and so is in town again? I just saw him the other day, I don't want to see him again." or you may hear, "I don't want to hear the same set, I doubt it will be anything new."
Sometimes the artist just does not want to burn themselves out in a certain market. They may also like to have at least 3 or 4 newly produced tracks before they play again if they only play their own tunes in their sets. The performer may also just be in town for the weekend, and wants to make as much money as possible, or just really loves to play.
When you limit your availability, you give yourself higher value. It makes people think that seeing you is a special occurrence since it doesn't happen often, and they are more likely to go to your show, and view you as a superstar.
I personally think it's cool to have a "Special Guest" listed. I love mystery and it reminds me of opening presents at Christmas. Sometimes you like the gift and freak out, and sometimes you just pretend to like it and smile at the effort.